Do you know where to find the voltage drop rule in the NEC? That’s a trick question.
The NEC has many rules. Some rules are mandatory [90.5(A)] (you must comply), and others are permissive [90.5(B)] (it’s allowed, but you don’t have to do it). But the NEC doesn’t have a rule for voltage drop.
Not everything in the NEC consists of rules. The NEC also has explanatory material that’s intended to help the Code user by providing information beyond the NEC’s purpose of protecting people and property. The material consists of Informational Notes [90.5(C)] and Informative Annexes [90.5(D)]. For voltage drop, there’s one Informational Note preceding 210.19(A), one following 310.15(A)(1), and one following 647.4(D).
The NEC provides the minimums necessary for safety, not design requirements that produce the most efficient operation. This is why the Informational Note in 310.15(A)(1) says, “Ampacities provided by this section do not take voltage drop into consideration.”